解决 SQLAlchemy 提示 Instance is not bound to a Session 错误的问题

在使用 SQLAlchemy 的过程中，有时会出现下列错误:

• Parent instance '<User at 0x2b45b53509d0>' is not bound to a Session; lazy load operation of attribute cannot proceed
• Instance '<User at 0x2b45b53509d0>' is not bound to a Session; attribute refresh operation cannot proceed

出现以上错误的原因是因为：session 已经被提交，导致操作的 model 对象已经不在当前 session 中了。 解决的办法就是：把对象重新加入到当前 session 中:

def foo(user):
    # do something...
    session.commit()

user = session.query(User).filter_by(name='Jim').first()
foo(user)
print user in session  # False
print user.name  # DetachedInstanceError: Instance <User at 0x2b45b53509d0> is not bound to a Session

user = session.merge(user)
print user in session  # True
print user.name  # Jim
session.refresh(user)
print user.name  # Eric